1. Technical Field
The present invention generally relates to mobile telecommunications, and in particular to a method and system for adaptively adjusting the downlink transmission rate to a mobile access terminal. More particularly, the present invention relates to a method and system that compensates for channel fading by periodically adjusting the downlink transmission rate in accordance with on-going packet error rate analysis.
2. Description of the Related Art
Mobile wireless access to the Internet and other communications networks is under rapid development. The development of mobile data communications capability is due to, and is modeled to some extent in light of, the success and advantages provided by the advent and development of mobile wireless telecommunications for voice communications. Several new air interface standards have been or are being developed to enable high speed wireless access to the Internet. These standards use fast feedback from a mobile terminal regarding channel conditions, which enable the downlink data rate to be quickly changed to compensate for signal fading. The technology used in these standards is generally known as high data rate (HDR) technology. One of these standards is referred to as 1xEV-DO, which has evolved into the industry standard IS-856.
HDR technology is typically implemented utilizing a combination of Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) technologies. In CDMA, all users transmit simultaneously over the entire allocated bandwidth utilizing specialized spreading codes. In TDMA, users take turns accessing the channel utilizing multiple time slots that are allocated for transmission over a given channel bandwidth. In this manner, TDMA enables a single frequency to support allocation of multiple, simultaneous data channels to access terminals. As utilized herein, an access terminal is a mobile device such as a laptop computer, palm-pilot, etc., that utilizes an air-interface to communicate with other terminals or network nodes via an access node, which is the air-interface network point-of-contact for any sending or receiving mobile terminal.
Existing HDR standards generally define two groups of channels, the forward channel (referred to hereinafter as the downlink channel) and the reverse channel (referred to hereinafter as the uplink channel). The downlink channel, which communicates voice and data from the access node to mobile access terminals, carries traffic, a pilot signal, and overhead information. The pilot and overhead channels establish system timing and station identity. The uplink channel, which communicates voice and data from the access terminal to the access node, carries both traffic and signaling.
Unlike voice or two-way multi-media sessions, “data” sessions (e.g. Internet file downloads) are highly asymmetrical, with the downlink (i.e. the channel information transmitted from the access node to the access terminal) capacity being a disproportionately critical parameter. On the downlink of an HDR system, data may be transmitted in a time division multiplexed manner. The downlink capacity in HDR systems is measured, at least in part, in terms of the data rate allocated to the access terminal. In HDR implementations, interference caused by signals from other cells is a determinative factor in the allocation of a particular data rate to a given access terminal. Excessive signal interference can cause a failure in decoding a packet delivered from the access node to the access terminal. Such a failure results in the need to re-transmit the packet, resulting in a diminished data transmission efficiency. Therefore, downlink data rate selection is a key parameter in maintaining the efficiency of a given HDR channel.
The various 3GPP and 3GPP2 HDR standards, which use time division multiplexing of the downlink (HSDPA and 1xEV-DO, for example), require methods for determining the appropriate data rate allocated to an access terminal downlink. Generally, this requires that the access terminal perform a measurement of the current channel conditions in terms of the signal-to-interference-plus-noise ratio (SINR), which is a ratio of the energy-per-chip interval (Ec) of the allocated channel to the outside spectral interference (Io). In 1xEV-DO, once the SINR is measured, the access terminal must update the access network with data rate control (DRC) requests that map to a set of data rates in bits-per-second (bps). It is the responsibility of the access terminal to select a data rate appropriate to the received SINR, such that the resultant packet error rate (PER) falls within certain limits specified in the applicable standard. In 1xEV-DO, the access node subsequently transmits data to the access terminal at the data rate specified by the DRC request. Therefore, in 1xEV-DO the data rate selection function typically resides in the access terminal. Once the access node has received the DRC request and determined that the access terminal should receive a packet, the access node transmits the packet over one or more time slots in accordance with the requested DRC rate.
Channel fading is a major source of channel signal strength fluctuations. So-called “slow fading” is caused by movement of the access terminal with respect to the access node (typically an RF transceiver station) resulting in interference in the air interface path between the access terminal and Access node due to changing physical topology (buildings, power lines, etc.). “Fast fading” is a phenomenon associated with collisions of multiple versions of the transmitted signal that arrive at the receiver at slightly different times and is typically characterized in terms of Doppler Effect and Rayleigh fading factors. HDR technology may account for channel fading by providing a built-in error factor into the downlink DRC request such that the data rate requested is a product of a very conservative estimate. Implementation of a downlink data rate based on such a conservative estimate results in wasted RF resources and reduced throughput. Alternatively, channel fading can be accounted for directly by modeling and predictively estimating the channel fading that will occur for a given nomadic access terminal. Companies that provide wireless mobile communications are adopting ray tracing and Doppler Effect tools that attempt to compute the effects of channel fading in a complicated environment. Such methods present daunting computational objectives which require substantial and costly hardware and software overhead since these methods directly or indirectly must account for access terminal speed, access terminal location within a given sector (with respect to an access node), and line-of-sight information between the access terminal and the access node.
It can therefore be appreciated that a need exists for an improved approach to compensating for channel fading in the allocation of downlink channels in a mobile wireless environment. The present invention addresses such a need.